Practical wisdom and the way we teach in the UK is that if you do not know the weight of the load, do not try to lift it but find the paperwork or something that tells you the weight first.
The issue of course being that the truck might lift the load six inches from the floor and you think it's okay, but as you lift higher the capacity reduces and you could end up tipping the truck and load if the weight is more than the truck can handle.